/*
 * 快慢指针判断链表是否有环
 * https://leetcode-cn.com/problems/linked-list-cycle/
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
bool hasCycle(struct ListNode *head) {
    struct ListNode *fast = head, *slow = head;
    while (fast != NULL && fast->next != NULL) {
        fast = fast->next->next;
        slow = slow->next;
        if (fast == slow) return true;
    }
    return false;
}